Circuit and method for driving alternatively electric loads with low impedance

Abstract: An electronic driver circuit for low-impedance loads, being of a type which comprises an input terminal (IN) to which a voltage signal (Vin) is applied for alternate transfer to an output, and a plurality of output terminals (OUTi), each connected to a corresponding electric load (2), further comprises, between the input terminal and the output terminals, a single operational amplifier (3) having multiple output stages (7), one for each output terminal (OUTi). The operational amplifier (3) is of the single-ended or fully differential multistage type and allows each load to be driven alternately by activation of the corresponding output stage (7).

Description: Field of the invention This invention relates to an electronic circuit for driving in an alternate fashion low-impedance electric loads according to the preamble of claim 1. The invention also concerns a method for driving low-impedance electric loads according to the preamble of claim 6. In particular, the invention relates to a circuit for driving low-impedance electric loads, being of a type which comprises an input terminal, whereto a voltage signal is applied for subsequent alternate transfer to an output, and a plurality of output terminals, each connected to a corresponding electric load. As is well known, many electronic applications involve the availability of integrated circuits capable of driving a plurality of low-impedance loads in an alternate fashion. A typical example of such applications is cellular phones, whereto an integrated circuit is incorporated which is to drive directly, but alternately, at least three sound transducers. In particular, these transducers include: a handset earpiece representing an electric load of approximately 100 Ohms and 350 nF; an approximately 30-ohm loudspeaker; and a second handset earpiece of about 600 Ohms. The moment it is operated, each transducer should be the only one to receive the electric signal. Background Art The prior art has already proposed some solutions to fill this demand. A first solution is illustrated schematically by Figure 1 of the accompanying drawings. This Figure shows a circuit which comprises an operational amplifier of the single-ended type having an input terminal and an output terminal. The output terminal is connected to a plurality of outputs OUTi of the circuit through respective selection switches SWi. A voltage signal Vin applied to the input terminal of the amplifier can be transferred alternately to a given output OUTi by driving its corresponding selection switch SWi. Each switch may be implemented with a CMOS transfer. However, this prior solution only holds when the impedance of each load at the output end is comparatively high. In fact, the selection switches associated with each load should have a power-on resistance Ron which is much lower than the impedance of the corresponding load, in order to fully transfer the electric signal. Assuming that a load of 30 Ohms is to be driven, the resistance Ron of the switch should then be less than 3 Ohms, and a MOS transistor of a huge size would be required to produce such a value. In addition, to design a circuit for operation on a power supply Vcc of just 3 Volts, p-channel MOS transistors with a W/L geometrical ratio of 50,000 to 1.2 and n-channel MOS transistors with a 10,000 to 1.2 ratio would have to be used. Due to such drawbacks, a second known solution, illustrated schematically in Figure 2, has been employed hitherto. This second solution basically provides for the switches to be shifted to somewhere ahead of the amplifier, specifically at the amplifier input end. This means, in essence, that an amplifier will be-associated with each of the outputs OUTi. While being an improvement on the previously described solution, this has a drawback in that an amplifier must be provided after each switch; accordingly, the solution is a costly one in terms of circuit layout and power consumption. A further solution is known from the US patent No. 4,177,432 which relates to a driver circuit according to the preamble of the annexed claim 1. However, this solution requires many compensation capacitances, one for each output stage. A similar solution is also disclosed in the abstract of the Japanese patent No. JP-A-52 026 948 which relates to a driver circuit including a compensation network for each output stage. The technical problem underlying this invention is to provide a driver circuit, and method therefor, which have uniquely simple structural and functional features and such that electric loads, even low-impedance loads, can be driven alternately, thereby overcoming the limitations of the prior solutions. Summary of the Invention The solutive idea on which this invention stands is one of using a single operational amplifier having multiple output stages which can be selected and operated in an alternate fashion. Based on this solutive idea, the technical problem is solved by a circuit as indicated above arid defined in the characterizing portion of the appended Claim 1. The technical problem is also solved by a driving method as defined in the characterizing portion of Claim 6. The features and advantages of the inventive circuit and method will be apparent from the following detailed description of an embodiment thereof, to be taken by way of example and not of limitation with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is a diagram illustrating schematically a driver circuit for electric loads according to the prior art; Figure 2 is a diagram illustrating schematically a second driver circuit for electric loads according to the prior art; Figure 3 is a diagram illustrating schematically an embodiment of the driver circuit of this invention; and Figure 4 is a diagram illustrating schematically a detail of the circuit of this invention. Detailed Description with reference in particular to Figure 3, generally and schematically shown at 1 is a drive circuit embodying the invention and adapted to power in an alternate fashion low-impedance electric loads, denoted by 2. The circuit 1 has an input terminal IN which is applied an electric voltage signal Vin to be transferred alternately to a selected output OUTi connected to one of the loads 2. In the example illustratively discussed herein, the circuit 1 has three outputs, namely: OUT1, OUT2, OUT3, each connected to a corresponding load 2. The circuit 1 comprises an operational amplifier 3 having a first or inverting (-) input, a second or non-inverting (+) input, a first output terminal OUT for setting the circuit gain, and a plurality of further output terminals OUTi, one for each load 2. The amplifier 3 is powered across a first voltage reference Vcc and a second reference, such as a signal ground GND. The input IN of the circuit 1 is connected to the inverting (-) input of the operational amplifier 3 through an impedor Z1. A constant voltage reference Vrif is maintained at the other, non-inverting (+) input of the amplifier 3. Advantageously, the operational amplifier 3 comprises three stages 5, 6 and 7 cascade connected to one another, namely: a first input stage 5 to which the aforesaid inverting (-) and non-inverting (+) inputs belong, an intermediate stage 6, and at least one output stage 7. Furthermore, the amplifier 3 is a single-ended type with nested-Miller compensation, as described in "A CMOS low-distortion fully differential power amplifier with double nested-Miller compensation", IEEE JSSC, July 1993, for example. However, the considerations set forth herein below may also apply to operational amplifiers of the fully differential type and to other forms of compensation as well. The amplifier 3 has an external feedback between the first output terminal OUT and the inverting (-) input, in which an impedor Z2 is connected. This feedback sets the gain Gam for the amplifier 3 as the ratio of the impedances of Z2 and Z1, Gam=-Z2/Z1. The feedback may be resistive, capacitive or a combination thereof. A first compensation capacitor Cc is connected between the output node OUT and the output of the input stage 5; that is, between the node OUT and the intermediate stage 6 input. The intermediate stage 6 comprises a pair of amplifiers 6a and 6b connected after the input stage 5 and having respective outputs Ua, Ub. These outputs Ua, Ub are connected to the input side of a plurality of stages 7a, 7b, 7c, one for each output terminal OUTi, through respective pairs of switches SW1a, SW1b; SW2a, SW2b; and SW3a, SW3b, that is one pair for each stage 7i. In addition, a corresponding compensation capacitor Cca, Ccb is connected between each output Ua, Ub of the intermediate stage 6 and the output node OUT of the amplifier 3. Each stage 7i comprises an output buffer incorporating a complementary pair of MOS transistors Ma, Mb, of the p-channel and n-channel type, respectively. The source terminal Sa of the first transistor Ma is connected to the supply voltage reference Vcc, whilst the source terminal Sb of the second transistor Mb is connected to the second voltage reference, i.e. the signal ground GND. The output Ua of the amplifier 6a is connected to the gate terminal Ga of the first transistor Ma via the switch SW1a. Another switch, SW1c, connects the gate Ga to the voltage reference Vcc. The output Ub of the amplifier 6b is connected to the gate terminal Gb of the second transistor Mb via the switch SW1b. A further switch, SW1d, connects the gate Gb to ground GND. The transistors Ma and Mb connect to each other at the node P through their respective drain terminals. The node P interconnecting the transistors Ma and Mb essentially represents the output of each stage 7i and is connected directly to one of the output terminals of the circuit 1. Also provided is a switch SWi between each node P of each stage 7i and the output terminal OUT of the amplifier 3. All the switches described can be implemented with CMOS transfer of minimal, or at least very small, area, since they are in series with no low impedance. The turning on/off of such switches is controlled by voltage signals generated by other circuitry, external of the circuit 1 and not shown because conventional. An exemplary embodiment of one of the output stages 7i of the amplifier 3 is illustrated in greater detail by Figure 4. Switches SWia, SWib, respectively connected to the outputs Ua and Ub of the intermediate stage 6, are each implemented by means of a complementary pair of MOS transistors, one of the p-channel and the other of the n-channel type. Such transistors have their respective source and drain terminals connected together. The gate terminal of the n-channel transistor receives an enable signal EN from the aforesaid external circuitry, while the gate terminal of the other, p-channel transistor is at the same time applied a signal EN in phase opposition to the enable signal. The switch SWi which connects the output node P to the first output terminal OUT is also implemented by a similar complementary pair of MOS transistors. On the other hand, for the switch SWic which connects the gate terminal Ga of the transistor Ma to the voltage supply reference Vc, a transistor Mc of the p-channel MOS type, having its source terminal connected to the voltage supply and drain terminal connected to the gate Ga, is preferred. The gate terminal of said transistor Mc receives, in turn, the enable signal EN. The other switch, SWid, which connects the gate terminal Gb of the transistor Mb to the signal ground GND is implemented by a transistor Md of the n-channel MOS type having its drain terminal connected to the gate Gb and its source terminal connected to ground. The gate terminal of said transistor Md is applied the signal EN being the complement of the enable signal EN. The operation of the circuit 1, as well as the driving method according to this invention will now be described briefly. Upon closing the switch pair SWia, SWib, the outputs Ua and Ub of the intermediate stage 6 become connected to one of the output stages 7a or 7b or 7c, and concurrently closing the switch SWi associated with the output P of that particular stage 7a or 7b or 7c, the connection to all the compensation capacitors Cc, Cca and Cb and the feedback impedor Z2 will be completed. The other output stages, on the contrary, are held open because the gate terminal Ga of the first output transistor Ma, of the p-channel type, is connected to the voltage supply reference Vc as a result of the switch SWic being closed; whilst the gate terminal Gb of the other output transistor, Mb of the n-channel type, is connected to ground as a result of the switch SWid being closed. Stated otherwise, the stages 7a or 7b or 7c are respectively selected by means of the switches SWia, SWib and SWi such that only one of them will be on to output power to a corresponding load 2, whilst the non-selected stages are turned off through the switches SWic and SWid. In this way, the voltage signal Vin at the input IN will be transferred to a load 2 connected to the output OUTi which has been selected by activation of the corresponding output stage 7i. Thus, the inventive method provides for each low-impedance load to be driven directly from an amplifier having a plurality of cascaded stages the last of which comprises a plurality of output stages 7a, 7b, 7c,... connected in parallel. Only one of these output stages will be activated or deactivated according to whether that particular output OUTi of the driver circuit is to be driven or not. The invention does solve the technical problem and affords a number of advantages, first among which is that the proposed solution allows electric loads having a low impedance to be driven in an efficient manner. In fact, the circuit of this invention includes a single operational amplifier, so that power requirements can be significantly less than with prior solutions. In addition, the overall area occupied by the circuit may be regarded as being much smaller than with prior solutions on account of the invention affording savings in layout that would be missed by prior solutions employing multiple amplifier, each involving the need to have their respective compensation capacitors and feedback impedors duplicated. Changes and modifications may be made unto the circuit described hereinabove within the scope of the invention as defined in the following claims.

Claims: An electronic driver circuit for low-impedance electric loads, comprising: a plurality of output terminals (OUTi) an input terminal (IN) whereto a voltage signal (Vin) is applied for transfer to a single output terminal (OUT) and for alternate transfer to one of the output terminals (OUTi), each connected to a corresponding electric load (2), a single-ended multistage operational amplifier (3) having a plurality of output stages ( 7), one for each output terminal (OUTi), connected between the input terminal (IN) and the output terminals (OUTi), characterized in that each of said plurality of output stages (7) comprises an output buffer (7a, 7b, 7c...) with a complementary pair of transistors (Ma, Mb) connected between a first voltage reference (Vc) and a second voltage reference (GND) and driven from an intermediate stage (6) of the amplifier (3), each output buffer having an output node (P) connected to the output terminal (OUT) of the operational amplifier (3) through a switch (SWi) and to a corresponding one of said output terminals (OUT i ). A circuit according to Claim 1, characterized in that said operational amplifier (3) is a fully differential multistage (5, 6, 7) type. A circuit according to Claim 2, characterized in that said operational amplifier (3) is a nested-Miller compensated type. A circuit according to Claim 1, characterized in that the control terminal (Ga) of the first (Ma) of said transistors (Ma, Mb) is connected to the first voltage reference (Vc) through a first switch (SWic) and the control terminal (Gb) of the second (Mb) of said transistors is connected to the second voltage reference (GND) through a second switch (Staid). A circuit according to Claim 1, characterized in that said intermediate stage (6) comprises at least one pair of amplifiers (6a, 6b), each having a respective output (Ua, Ub), and that each stage (7a, 7b, 7c,...) in said plurality of output stages (7) is connected to the outputs (Ua, Ub) of the intermediate stage (6) through a respective switch pair (SWia, SWib). A method for driving in an alternate fashion low-impedance loads comprising the steps of: connecting each load (2) to a corresponding output terminal (OUTi) of a driver circuit (1) having an input terminal (IN) whereto a voltage signal (Vin) is applied for transfer to a single output terminal (OUTi), using a single multistage (5, 6, 7) amplifier (3) having a plurality of output stages (7), one for each output terminal (OUTi), activating that output stage (7) which is associated with the load, to be driven, characterized in that it further comprises the steps of: selectively connecting the buffer output (P) of the active stage (7) to an output terminal (OUT) through a switch (SW i ) such that the activation and driving phase of said output stage (7) is controlled from an intermediate stage (6) of said amplifier (3).

Inventors: NICOLLINI GERMANO, PERNICI SERGIO, NICOLLINI, GERMANO, PERNICI, SERGIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H04R3
IPC codes: H03F3